This invention relates generally to a process for charging magnetic toner compositions, and more specifically to a process for inductively charging magnetic toner compositions which charge is dependent on the polarity and magnitude of the development field rather than on the nature of the compositions used in the developer mixture, and the triboelectric properties of the carrier and toner employed.
The formation and development of electrophotographic images, and more specifically xerographic images is well known in the art as described for example in U.S. Pat. No. 2,297,691. In one common technique of development, carrier materials and fine insulating toner powder particles are cascaded over the electric potential pattern bearing member. The powder is triboelectrically charged to a certain polarity and magnitude and deposits preferentially in regions of the member surface where there is a preponderence of charge of opposite polarity. Generally, the triboelectric charge is caused by the presence of carrier beads in the developer mix. In another form of development known as magnetic brush development, magnetic carriers are employed, reference U.S. Pat. No. 3,641,980. In this method magnetic forces are employed for the purpose of causing the toner to deposit on the imaging member. In comparison to cascade development, magnetic brush development fills in solid areas better, is more compact, and does not depend on gravity to present the toner to the surface, a factor which allows freedom in locating the developer station. Cascade development is described in U.S. Pat. No. 2,618,552, while magnetic brush development is described, for example in U.S. Pat. Nos. 3,641,980, 2,874,063, 3,251,706 and 3,357,402. Other development methods include powder cloud development, as described in U.S. Pat. No. 2,221,776, and touchdown development, as described in U.S. Pat. No. 3,166,432. In the '432 patent there is described the use of a conductive one component developer (toner and no carrier particles) for developing electrostatic charge patterns by bringing a conductive support member bearing a layer of fine conductive toner particles into contact with the charge pattern bearing member. In this method the toner is held to the support member primarily by Van der Waals forces, and the conductive support member is held at a bias potential during development. This technique is apparently well suited for filling solid areas, and as an additional advantage requires only one component in the developer material.
In the conventional cascade development technique the toner carrier combination or developer has a definite charge polarity, and triboelectric relationship. Positive and negatively charged images cannot easily be made visible with the same developer, and further the images provided from such developers can be hollow in that solid areas are not filled, resulting in low development quality. The triboelectric properties of the toner while necessary to development can cause problems, for example, uneven charging of the toners causes background deposits as the uneven forces between carrier and toner result in varying threshold levels from toner particles to toner particles. Further since the toner retains its charge for long periods of time, any toner that escapes the development zone and enters into other parts of the apparatus can cause mechanical problems. Magnetic brush development, while it overcomes some of the problems encountered in cascade development, is in some instances less efficient in that it still required triboelectric toners which have the concomitant problems mentioned above. Further because of the mechanical brushing action and other electrical characteristics magnetic brush development can results in high background deposition and poor machine latitude.
There has also been described in the prior art magnetic development materials and systems wherein carriers are not utilized, that is, a one component type system. One such system is described in Wilson U.S. Pat. No. 2,846,333 which discloses the use of a magnetic brush to apply toner particles formed of magnetites and resin materials to develop electrostatic latent images. One difficulty encountered with this process is that the relatively high electrical conductivity of the toner renders electrostatic transfer rather difficult. Also, as described in U.S. Pat. No. 3,909,258, electrostatic development is accomplished by utilizing a magnetic brush without carrier, the particular toner employed being the toner of U.S. Pat. No. 3,639,245 which is a dry toner particle having specific electric conductivity.
While all of the above methods have certain advantages in particular situations, each has some disadvantages which have an adverse effect on their utility in development systems. Also in conventional magnetic brush systems wherein two component materials are used, that is, toner and carrier, the charge present on the toner is dependent upon the triboelectric relationship between the toner and carrier particles. Further there have been encountered in magnetic brush systems problems with regard to maintaining over a period of time sufficient adhesion between carrier and toner. In an aged developer, that is, for example, one that has been used in a xerographic imaging device for causing the development of images, toner separation occurs prematurely which causes the quality of the image being developed to be much lower than when the developer material was fresh. Also, the triboelectric charging characteristics of the developer are reduced, that is, the overall charge in microcoulombs per gram contained on the toner particles as the developer ages is less than it should be, thereby adversely affecting copy quality.